SANAA, Yemen — It was an awkward moment for Yemen’s acting head of state.

In a meeting Monday, top opposition leaders referred to Abed Rabbo Mansour Hadi as Mr. President. A senior ruling party official, deeply loyal to embattled President Ali Abdullah Saleh, retorted that Hadi was only the vice president.

“I am the acting president according to the Constitution,” Hadi replied with a smile, trying to strike a middle ground, according to two senior officials who were at the meeting at Hadi’s house.

For 17 years, Hadi served in relative anonymity as Saleh’s vice president. Today, he is widely perceived as someone who might guide his strategic Middle Eastern nation through a peaceful transition period that would end Saleh’s nearly 33-year-long rule and usher in a new political era.

But the question on many minds is whether Hadi has the strength and will to take on Saleh’s allies — including his son and nephews, who have remained inside Yemen even as the president recuperates in neighboring Saudi Arabia from severe injuries sustained in a June 3 attack on his presidential compound.

Saleh’s relatives continue to wield enormous influence in his absence. This week, security forces loyal to Saleh’s son and nephews were posted in front of Hadi’s residence, a sign that many interpreted as a warning to Hadi not to cross any red lines imposed by Saleh and his family.

“He is in the middle of the hammer and the bench,” said Sultan al-Atwani, a senior leader of Yemen’s traditional political opposition.

On the one hand, Hadi is facing mounting pressure from Yemen’s various centers of power — from youth activists to traditional opposition parties to powerful tribal leaders — to formally assume presidential authority. On the other, Atwani said, “parts of the regime, the sons and nephews, do not see him as legitimate. They see him only as the vice president until the president comes back.”

Compared with other top Yemeni leaders, Hadi is relatively unknown to the United States and its allies. But in a nation deeply riven along political, tribal and geographical lines, American officials appear to regard Hadi as a unifying figure who might appease the various competing interest groups, ensuring a smooth handover of power until elections can be held.

And having spent much of his career in the military, including a stint as defense minister, Hadi could potentially provide a positive answer to the most pressing question being posed by the U.S. government about Yemen’s future: Will Saleh’s successor be as committed to fighting Yemen’s emboldened Islamist militants and an ambitious al-Qaeda branch that has targeted American soil?

U.S. officials say they are encouraged that Hadi is reaching out to the opposition. “We believe that there is no time to lose in moving on to the democratic future that Yemen deserves,” State Department spokeswoman Victoria Nuland told reporters Thursday, referring to Hadi’s efforts to promote a political dialogue.

Hadi’s office did not respond to repeated requests for an interview.

How loyal will Hadi be?

Hadi, 66, was born in the southern province of Abyan. Educated in a military school in Aden, when South Yemen was a British protectorate, he later went to Britain, Egypt and Russia for further military studies. Afterward, he rose through the ranks of the South Yemen military. An avid reader, he speaks fluent Russian and good English. In contrast, Saleh did not receive much education and entered the military in his teens.

In 1986, when civil conflict erupted between two factions of the ruling socialists, Hadi left for North Yemen. There he became close to Saleh, especially after North and South Yemen merged in 1990. In 1994, when civil war broke out, Hadi was appointed defense minister and played an important role in defeating the southern socialists. Later that year, Saleh appointed Hadi as his vice president; in 1997, he was promoted to the rank of lieutenant general.

The mere fact that Hadi has been able to remain vice president under Saleh for so long speaks to his skill at political survival. Saleh, one of the Middle East’s master political tacticians, swiftly marginalized anyone who he felt threatened his grip on power.

“He listens very well,” said Ali Saif Hassan, a political analyst who knows Hadi socially.

In the four months since Yemen’s populist revolt galvanized the country, fissures in Yemen’s political landscape have deepened. Government soldiers have fought tribal militias, turning parts of this sprawling capital into a war zone.

The military and security forces are divided in their loyalties to Saleh, his son and nephews — and to Maj. Gen. Ali Mohsen al-Ahmar, once Saleh’s closest ally, who defected in March after snipers loyal to Saleh killed more than 70 protesters on a single day. There is also infighting within Yemen’s largest tribal confederation, the Hashid, which has long controlled Yemen’s government and economy.

Hadi’s non-threatening nature and background appeal to all sides.

His origins would appease southerners who have long wanted someone from their region to become president after Saleh, who is a northerner. Hadi is neither a Hashid nor a Sanhan, the clan of Saleh, Mohsen and other major figures in the ruling party and military, and hence acceptable by the opposition. Saleh opponents also like that he is well educated and shows no signs of tribal loyalties.

“He’s not looking to hold on to power,” said Hassan Zaid, a top opposition leader.

Hadi is perceived as being close to Mohsen; they worked together during the 1994 civil war. But Hadi has not shown any signs so far that he would side with Mohsen against Saleh. In fact, publicly he has rejected any suggestions that he replace Saleh, predicting that the president will return home within days. Many of Saleh’s allies in the government are convinced that Hadi will remain loyal.

“He is one of the closest people to the president,” said Ahmed Bin Daghr, assistant secretary general of the General People’s Congress, the ruling party.

Pressure is building on Hadi to make a choice.

In recent days, thousands have protested in front of Hadi’s residence, and many opposition leaders hope that Hadi will accept the idea of becoming an interim president, with elections to follow, under a constitutional procedure allowed when the president is deemed incapable of ruling. Others want him to create a transitional presidential council.

This week, Sadiq al-Ahmar, the head of Yemen’s most powerful tribal family, wrote a letter urging Hadi to formally declare himself president. He promised that his own forces would deal with Saleh’s sons and nephews, whom his militias challenged in pitched battles against government forces this month.

“I told him we would give him whatever he needs, and much more,” Ahmar said in an interview in his residential compound, shattered by rockets and shelling. “He has to take action.”

Staff writers Joby Warrick and Karen DeYoung in Washington and special correspondent Ali al-Mujahed and another special correspondent in Sanaa contributed to this report.